Monoterpenes and related compounds (monoterpenoid alcohols, ketones, etc.) are a structurally diverse group of chemicals that share a common two-isoprene (2-methylbuta-1,3-diene) basic structure. They have broad uses industrially as solvents (e.g. turpentine) or as flavorants or scents. They are produced naturally mostly by plants: in the rinds of citrus fruits, or leaves of some species, or in the woody tissue of coniferous trees. For example, pine tree resin contains high monoterpene levels and is a major source of turpentine.
Some monoterpenes and monoterpenoids are chiral, that is, they have a “handedness.” Molecules that are structural “mirror images” of each other are called enantiomers. Enantiomers are usually referred to as “(+)” and “(−)”; “R” and “S”, or “D” and “L”. Although different enantiomers are chemically identical, then biochemical or physiological properties can be very different. For example, (+)-limonene has a citrus smell, while (−)-limonene smells like turpentine. A mixture of equal amounts of both enantiomers of a chiral compound is called a racemic mixture.
Pine bark beetles are closely linked to monoterpenes because they live in the phloem of pine trees, where they are exposed to and must deal with high monoterpene concentrations. Also, most species rely on monoterpenoid-based pheromonal communication to successfully attack a host tree. The enantiomeric composition of the pheromone blend is important. For example, the aggregation pheromone of western populations of Ips pini consists of ˜95% (−)-ipsdienol (2-methyl-6-methylene-2,7-octadien-4-ol) while a racemic mixture is not attractive at all. In contrast populations of I. pini from eastern North America use racemic ipsdienol as a pheromone. A related species endemic to the southeast U.S., I. calligraphus, is attracted to (−)-ipsdienol, but dispersed by (+)-ipsdienol.
There are a number of commercially important pine bark beetle species. For example, Ips calligraphus is distributed across mostly eastern North America. As mentioned above, I. calligraphus responds to (R)-(−)-ipsdienol. Like other scolytids, Ips spp. periodically cause loss of wood (cut wood and sometimes standing trees) over extensive areas. Their galleries do not affect the structural properties of the wood significantly, but may render it useless for veneer or furniture making. However, they tend to be less aggressive and less host-specific than Dendroctonus spp. They mostly breed in slash, or in broken, fallen or dying trees, but I. calligraphus can, under favorable, conditions make successful primary attacks on healthy Pinus stands.
I. calligraphus forms part of the so-called “southern pine bark beetle guild” (including also Dendroctonus frontalis, D. terebrans, I. grandicollis and I. avulsus), which attacks disturbed Pinus spp, in southeastern USA (e.g. disturbance by lightning strike, attacks by defoliating insects) and also causes economic problems by infesting freshly cut logs and pulpwood and introducing bluestain fungi. Published information on I. calligraphus as a pest relates almost exclusively to this area and to the Caribbean. It is considered only a secondary pest in California. Subsp. ponderosae, though reportedly widespread in western USA, is not considered as a pest there.
Ips confusus is distributed across the western US. I. confusus infests pinyon pine trees and causes periodic, severe, widespread damage. This species responds to (+)-ipsdienol and ipsenol. They mostly breed in slash, or in broken, fallen or drying trees. In this way, I. confusus can kill pinyon pines in southwest USA, when outbreaks start on trees that are damaged or uprooted in land-clearance schemes. I. paraconfusus is of greater practical importance because it attacks the timber tree P. ponderosa, killing saplings and young trees up to about 65 cm in diameter. Outbreaks develop on recently cut wood and spread to nearby living trees. This is reflected by a much greater number of publications on I. paraconfusus than on I. confusus. Top-killing by I. paraconfusus can contribute to outbreaks of the more dangerous pest Dendrocionus brevicomis. 
Ips typographus is distributed across northern Europe and Asia. This species responds to ipsenol as an anti-aggregation pheromone. I. typographus is the most destructive species of the genus Ips, and probably the most serious pest on spruce in Europe. There are records of outbreaks dating from the eighteenth century. Those outbreaks resulted in the loss of million of cubic meters of wood.